A continuing demand exists for a simple, efficient and inexpensive process which can reliably provide water of a desired purity, in equipment which requires a minimum of maintenance. In particular, it would be desirable to improve efficiency of feed water usage, and lower both operating costs and capital costs for high purity water systems, as is required in various industries, such as semiconductors, pharmaceuticals, biotechnology, steam-electric power plants, and nuclear power plant operations.
In most water treatment systems for the aforementioned industries, the plant design and operational parameters generally are tied to final concentrations (usually expressed as total dissolved solids, or "TDS") which are tolerable in selected equipment with respect to the solubility limits of the sparingly soluble species present. In particular, silica, calcium sulfate, and barium sulfate often limit final concentrations achievable. In many cases, including many nuclear power plants and many ultrapure water plant operations, boron or other compounds of similarly acting ampholytes have a relatively low rejection across membranes in conventionally operated RO systems, and may dictate design or operating limitations. More commonly, the presence of such compounds result in sufficiently poor reverse osmosis product water, known as permeate, that additional post RO treatment is required to produce an acceptably pure water. In any event, to avoid scale formation and resulting decreases in membrane thruput, as well as potential deleterious effects on membrane life, the design and operation of a membrane based water treatment plant must recognize the possibility of silica and other types of scale formation, and must limit water recovery rates and operational practices accordingly. In fact, typical RO plant experience has been that declines in permeate flow rates, or deterioration of permeate quality, or increasing pressure drop across the membrane, require chemical cleaning of the membrane at regular intervals. Such cleaning has been historically required because of membrane scaling, particulate fouling, or biofouling, or some combination thereof. Because of the cost, inconvenience, and production losses resulting from such membrane cleaning cycles, it would be advantageous to lengthen the time between required chemical cleaning events as long as possible, while nevertheless efficiently rejecting undesirable ionic species and reliably achieving production of high purity permeate.
Since the introduction and near universal adoption of thin film composite membranes in the mid to late 1980s, the improvements in RO technology have been evolutionary in nature. Operating pressure needed to achieve desired rejection and flux (permeate production rate per unit of membrane surface area, commonly expressed as gallons per square foot of membrane per day, or liters per square meter per day) has been slowly reduced, while average rejection of thin film composite membrane has improved incrementally.
Historically, brackish water RO systems have been limited in their allowable recovery and flux rates by the scaling and fouling tendencies of the feedwater. It would be desirable to reduce the scaling and fouling tendencies of brackish feedwater to the point where recovery limits would be dictated by osmotic pressure, and where flux rates can be increased substantially, compared to limits of conventional brackish water RO systems.
From a typical end user's point of view, several areas of improvement in RO technology--chlorine tolerance being one of them--are still sought. Thin film composite membranes, at least partly due to their surface charge and characteristics, are relatively prone to biological and particulate fouling. With certain feedwaters, particularly from surface water sources, membrane fouling and the frequent cleaning required to combat fouling can present some arduous, costly, and time-consuming operational challenges.
It is known that rejection of weakly ionized species, such as total organic carbon ("TOC"), silica, boron, and the like, is significantly lower than rejections for strongly ionized species as sodium, chloride, etc. Since the efficiency of post-RO ion exchange is largely determined by the level of the weak anions present in the RO permeate, it would be advantageous to remove (reject) as many weak anions as possible in the RO unit operation. In other words, by removing (rejecting) more silica (and boron) in the RO step, a higher throughput is achievable in the ion-exchange unit operation that follows the RO unit.
With the exception of an RO process disclosed in U.S. Pat. No. 4,574,049, issued Mar. 4, 1986 to Pittner for a REVERSE OSMOSIS SYSTEM, which reveals a double pass (product staged) RO system design, carbon dioxide typically represents the largest fraction of the anion load in RO permeate. However, a multiple pass RO configuration provides very little benefit under conventional RO system operating conditions, since the carbon dioxide content of permeate stays at the same (absolute) level and represents an even bigger fraction of the anion load. High rejection of weak anions in a single pass RO system is, therefore, considered to be another area where significant improvement is still sought.
In addition to increasing the rejection of the weakly ionized species, the increased rejection of strongly ionized species is also desired.
Recovery rate, or volumetric efficiency, is another parameter where improvements in RO system performance would be advantageous. A typical RO system operates at about 75 percent recovery, where only 75 percent of the incoming feed to RO is used beneficially, and the rest (25 percent) is discharged. With water becoming both more scarce and more costly throughout the world, increasing the maximum achievable recovery rate in an RO system is an important goal.
Increasing the operating flux is always important to end users, as increased flux reduces capital costs.
Simplification and cost reduction for post-RO unit operations is also sought by end users. This is because allowable levels of impurities in ultrapure water have continually decreased with the ever tightening design rules in semiconductor device geometry. Thus, lower contaminant levels in the ultrapure water system are required. As a result, the cost and complexity of the post-RO system components have dramatically grown in recent years.
High purity water processing procedures and the hardware required for carrying them out are complex and expensive. In fact, the regenerable mixed bed ion exchange system represents, by far, the most expensive (and complicated) single unit operation/process in the entire ultrapure water treatment system. Thus, significant improvement in the characteristics of the RO treated water would appreciably reduce the overall ultrapure water system cost and complexity.
I am aware of various attempts, some in high purity water treatment applications and some in wastewater treatment applications, in which an effort has been made to improve the efficiency of the rejection of certain ions which are sparingly soluble in aqueous solution at neutral or near neutral pH. Such attempts are largely characterized by conventional hardness removal and then raising the pH with chemical addition. One such method is shown in U.S. Pat. No. 5,250,185, issued Oct. 5, 1993 to Tao, et al., for REDUCING AQUEOUS BORON CONCENTRATIONS WITH REVERSE OSMOSIS MEMBRANES OPERATING AT HIGH PH. In a preferred embodiment, his invention provides use of a conventional zeolite softener followed by a weak acid cation ion-exchanger operated in sodium form to remove divalent cations. Due to both equipment limitations and to process design considerations, his pretreatment steps are followed by the somewhat costly and otherwise undesirable step of dosing the feedwater with a scale inhibitor to further prevent hardness scales from forming. Also, although his method does provide a simultaneous hardness and alkalinity removal step, which is of benefit in many types of applications which are of interest to me, his method does not provide for a high efficiency in that removal step, as is evidenced by the fact that two additional downstream softening steps are required in his process. Moreover, his application pertains to, and is described and claimed with respect to oil field produced waters containing hydrocarbon compounds (containing carbon and hydrogen only, and generally not ionizable), whereas in applications which are of interest to me, such compounds are almost totally lacking. In applications of primary interest to me, a variety of naturally occurring organic acid such as humic and fulvic acids are present, particularly in surface waters presented for treatment.
Also, a method used in high purity water applications is disclosed in Japanese KOKAI No. Sho 58-112890, Published Jun. 29, 1984 by Yokoyama, et al., for a METHOD OF DESALINATION WITH A REVERSE OSMOSIS MEMBRANE UNIT. His examples show reverse osmosis units utilizing a pretreatment process of strong acid cation exchange resin ("SAC") for softening in one example, and without softening in the other example. While his process will work for certain feedwaters, it does not teach how operation at higher pH levels may be employed while still avoiding scaling of RO membranes.
In order to better understand my process; it is useful to understand some basic water chemistry principles. With respect to calcium carbonate (CaCO.sub.3), for example, the likelihood of occurrence of precipitation on an RO membrane in the final reject zone may be predicted by use of the Langelier Index, sometimes known as the Langelier Saturation Index (LSI). See the Nalco Water Handbook, copyright 1979, by McGraw-Hill. This index is generally formulated as follows:
LSI=pH.sub.reject -pH.sub.s PA1 pCa=-log of Ca.sup.++ ion concentration (moles/liter) PA1 pAlk=-log of HCO.sub.3.sup.- ion conc. (moles/liter) PA1 C=a constant based on total ionic strength and temperature of the RO reject PA1 allows the removal of hardness and alkalinity from a selected feedwater to be done in a simple, direct manner; PA1 has a minimum of unit process requirements; minimize or avoid complex chemical feed systems; PA1 requires less physical space than existing technology water treatment plants; PA1 is easy to construct, to start, and to service; PA1 has high efficiency rates, that is, they provide high product water outputs relative to the quantity of feedwater input to the water treatment plant; PA1 in conjunction with the preceding object, provide lower unit costs to the water treatment plant operator and thus to the water user, than is presently the case; PA1 in conjunction with the just mentioned object, results in less chemical usage than in most water treatment facilities, by virtually eliminating use of some types of heretofore commonly used chemical additives, particularly scale inhibitors.
where pH.sub.s =the pH at saturation of CaCO.sub.3 (reject) and pH.sub.s =pCa+pAlk+C
and wherein:
In a given RO reject water, in order to avoid carbonate scaling, it most preferable to keep the LSI negative, i.e. in a condition so that CaCO.sub.3 will dissolve. However, in the field, it has been found that under some conditions, with use of certain types of anti-scalant additives, an LSI of up to about +1.5 can be tolerated, without CaCO.sub.3 scale formation resulting. In any event, at the pH of any given RO reject, pH.sub.s must be minimized in order to avoid undesirable scale formation. To put this into perspective, consider that in any RO pretreatment operation, it can be anticipated that there will always be at least some leakage of calcium from the softening step. Thus, depending upon the raw feedwater hardness and the pretreatment process scheme practiced, a lower limit on the achievable value of the pCa term, due to the concentration of the Ca.sup.++ ion present in the treated RO feedwater, can be anticipated. Furthermore, in all events, the value of C is fixed by the total ionic strength and by the temperature. Thus, to keep the LSI in an acceptable range--in order to provide scale free RO operation--the leakage of calcium (as well as other hardness such as magnesium) becomes a critical factor. The Tao et al. patent, identified above, approaches this problem by providing various types of softeners in series. Specifically, he simply accepts the inevitably high capital and operating costs associated therewith. Yokoyama, on the other hand, evidently decided to limit RO operation to a pH which is consistent with the degree of calcium removal. When he operates with RO reject at a pH of 9, assuming 0.1 ppm of Ca.sup.++ leakage from the ion exchange train disclosed, and a concentration factor of 5 ("5.times.") in the RO, his RO operation may be expected to provide an RO reject with an LSI of about -0.5. That LSI is acceptable for non-scaling operation, with or without scale inhibitors. However, if the pH in Yokoyama's example were increased to 11, for example, given the same pretreatment method, an LSI of about +2.4 might be expected. In such a case, the Langelier Saturation Index of the reject water would be well above the level where current anti-scalants have the ability to provide scale free RO operation.
Thus, for the most part, the prior art methods known to me have one or more of the following shortcomings: (a) they do not reliably achieve the extremely low hardness and non-hydroxide alkalinity levels necessary for essentially scale free operation at very high pH levels; (b) they rely on redundant and expensive capital equipment, with attendant operating costs, to minimize hardness leakage, (c) they depend primarily on hardness reduction to reduce the LSI of the RO reject (and do not include provisions for high efficiency dealkalization), and (d) they rely on anti-scaling additives to prevent scale formation. Thus, the advantages of my simple treatment process which exploits (a) hardness removal to very low residual levels, and (b) efficient dealkalization, to allow extended trouble free RO operation at high pH levels, are important and self-evident.
Moreover, because of upper concentration factor limits due to the tendency of scale to form, RO systems are often unable to use about twenty five (25%) or more of the raw feedwater. Also, at recoveries levels greater than approximately seventy five percent (75%) or somewhat lower, depending upon raw water chemistry, the control of chemical scaling and biological fouling in conventional RO systems becomes almost unmanageably difficult when trying to achieve long run times. Therefore, widespread commercial use of RO systems with water recovery in excess of about seventy five percent (75%) has not been accomplished.
As water is becoming increasingly expensive, or in short supply, or both, it would be desirable to increase the ratio of treated product water to raw water feed in RO systems. Therefore, it can be appreciated that it would be desirable to achieve reduced costs of water treatment by enabling water treatment at higher overall recovery rates rates than is commonly achieved today. Finally, it would be clearly desirable to meet such increasingly difficult water treatment objectives with better system availability and longer run times than is commonly achieved today.
In so far as I am aware, no one heretofore has thought it feasible to operate a reverse osmosis based water treatment system at higher than about pH 9, in continuous, sustainable, long term operations to produce a highly purified treated water product. The conventional engineering approach has been to design around or battle scale formation, by use of moderate pH, by limiting final concentration and resulting water recovery, by use of chemical additives. Historically, cellulose acetate membranes were limited in operation to a pH range of roughly 4 to 7. Newer polyamide and thin-film-composite type membranes have traditionally been operated in the pH range of roughly from about 4 to about 8. Although higher pH operation has occasionally been attempted for a few special purposes, it has usually been in non-silica related applications. And, although higher pH operation has been utilized in second pass RO applications where silica was of concern, in so far as I am aware, it has only been accomplished after a first pass RO operation with a neutral or near neutral pH of operation. In those cases where organics are of specific concern, then the pH may often range to below 5, and preferably, below 4.
In contrast to prior art methods for water treatment, the method taught herein uses the essential design philosophy of virtually eliminating any possible occurrence of scaling phenomenon during first pass operation at the maximum feasible pH using the available membranes, while maintaining the desired concentration factor, and taking the benefit of water recovery that results.